The code to support it is starting to become increasingly complicated
and the number of problem found is a bit alarming.
We'll reinclude it later after a bit more testing and hopefully some
simplifications.
The dependencies on library artifacts are now properly setup to point
to the files in _build/install/...
Moreorver, private interfaces are now only visible inside the library
itself and are only allowed for private libraries. When a project
defines multiple packages, this ensures that the visibility when all
packages are built simultaneously and when they are installed one by
one.
We can relax these restrictions later with a bit more work and a clear
definition of where private modules should be visible.
Add a field "public_interfaces" to library stanza listing which modules are public.
Private modules won't be accessible outside the scope where the library is defined.